1. Field of the Invention
The present invention relates to the amplification of differential audio signals, such as those produced by high-quality microphones.
2. Description of the Related Art
Differential signaling is a commonly-used technique for transmitting information, whereby two complementary but opposite signals are sent on two separate, and typically balanced, transmission lines—possibly in a single cable. The motivation for employing differential signaling is invariably due to the need to suppress the effect of noise on the cabling used to transmit the information. For instance, even though the level of a signal transmitted over a cable may be reasonably large, say of the order of 100 millivolts, the length of the cable itself may cause the introduction of a significant degree of noise into both lines—in some cases up to ten volts of noise. By transmitting information in differential mode, where the signals are of opposite polarity to one another, the noise will however manifest as a common-mode signal.
The standard approach to recovering the original signal is to subtract the two signals on each line from one another in a pre-amplification stage, thereby rejecting the common-mode noise present on the two lines. Such an approach is commonly taken in high-quality microphone pre-amplifiers, particularly in mixing consoles of the known type, which pre-amplifiers also provide controllable gain.
Problems exist, however, with this approach to recovering the differential signal whilst maintaining common-mode rejection. Due to the inherent topology of circuits presently used in microphone amplifiers, there is no concept of electrical ground at the point in the circuit where gain is controlled. The introduction of variable gain elements that require a connection to ground, such as a digital-to-analog converter (DAC), is therefore not possible. Thus, up until now it has not been feasible to employ commodity DACs in differential signal amplifiers to allow digitally controllable gain.